High brightness broadband coherent light sources have many applications in medicine, spectroscopy, microscopy, ranging, sensing and metrology. Such sources need to be highly robust, have long term stability, and also comprise a minimal component count with a high degree of optical integration for mass market applications. Broadband light sources based on frequency broadening or supercontinuum generation in highly nonlinear fibers are particularly useful. When used in conjunction with short pulse fiber lasers, an all-fiber system construction is possible for supercontinuum generation which results in benefits such as greatly simplified manufacturing routines, low cost and high levels of thermo-mechanical stability.
Fiber based supercontinuum sources can produce spectral output from the UV to the mid-IR and have attracted a vast amount of research in the last few years, see for example J. M. Dudley et al., ‘Supercontinuum generation in optical fibers’, Cambridge University Press (2010). To reach the mid-IR, for example the wavelength range from about 2.5-10.0 μm, soft glasses or heavy metal oxide glasses may be implemented for supercontinuum generation, as recently reviewed by J. H. V. Price et al., ‘Supercontinuum generation and nonlinearity in soft glass fibers’, in chapter VI of J. M. Dudley et al., ‘Supercontinuum generation in optical fibers’, Cambridge University Press (2010). Such fiber based mid-IR sources operating in the mid-IR can potentially replace more established optical parametric oscillators (OPOs), amplifiers (OPAs) and generators (OPGs) and are therefore of considerable interest.
Highly nonlinear fibers based on silica glass have already reached a relatively high level of maturity. To reduce the pulse energy requirements for supercontinuum generation, highly nonlinear silica fibers with extremely small cores are beneficial. For example, silica based highly nonlinear fibers and in particular photonic crystal fibers were recently described in Dong et al., ‘Ultra high numerical aperture optical fibers’, U.S. Pat. No. 7,715,672, where the additional use of highly Germania doped central core sections was further suggested to lower the pulse energy requirements for supercontinuum generation. Indeed silica fiber based supercontinuum sources employing short pulse fiber sources were, for example, described in T. Hori, ‘Studies on Ultrawideband Supercontinuum Generation by Use of Ultrashort Pulse and Optical Fibers’, Ph.D. Thesis, Nagoya University, Japan (2005). These all-fiber supercontinuum sources were operated using short pulse lasers emitting at wavelengths near 1560 nm and used highly nonlinear silica fibers with high levels of Germania concentration inside the core. Such all fiber sources were also shown to produce supercontinua with high levels of coherence and were used in the demonstration of ultra-low noise frequency comb sources in W. C. Swann et al., “Fiber-laser frequency combs with subhertz relative bandwidths”, Opt. Lett., vol. 31, pp. 3046-3048 (2006). Low noise frequency comb sources operating with laser sources emitting near 1550 nm can operate at repetition rates in the range from 50-1000 MHz. The upper limit is generally governed by design constraints of the laser sources implemented. The lower limit is governed by mechanical stability considerations.
There still remains a need for low noise supercontinuum sources that can operate at repetition rates >1 GHz, particularly at wavelengths near 1550 nm. There also still remains a need for low noise supercontinuum sources that can operate with short pulse laser sources operating at wavelengths >1700 nm and produce broad coherent spectral coverage extended to the mid IR. Also, there still remains a need for low noise highly coherent supercontinuum sources based on soft glasses or highly nonlinear waveguides.